True airspeed measuring apparatus

ABSTRACT

Apparatus relying on the adiabatic temperature rise due to stopping a moving stream of air for determining the velocity of that air. Temperature sensors are mounted at opposite ends of a rotating arm for eliminating the need to measure static temperature and for providing a linear relationship between velocity and temperature measurements.

ite States atent 1 Hazen May 29,1973

TRUE AHRSPEED MEASURING APPARATUS Inventor: Edward J. Hazen, WoodcliffLake,

Assignee: The Bendix Corporation, Teterboro, NJ.

Filed: Dec. 21, 1970 Appl. No.: 100,241

US. Cl ..73/l81, 73/178 H, 73/189 Int. Cl. ..G01c 21/10 Field of Search..73/181, 178 H, 399,

References Cited UNITED STATES PATENTS 1/1963 Garbell ..73/178 HBechberger et a1... 73/181 X Clousing et al ..73l18l PrimaryExaminer-Donald O. Woodiel Attorney-Anthony F. Cuoco and Plante, Hartz,Smith and Thompson [57] ABSTRACT Apparatus relying on the adiabatictemperature rise due to stopping a moving stream of air for determiningthe velocity of that air. Temperature sensors are mounted at oppositeends of a rotating arm for eliminating the need to measure statictemperature and for providing a linear relationship between velocity andtemperature measurements.

7 Claims, 5 Drawing Figures PATENTEL M29 1973 3, 73 5 63 5 SHEET 1 or 3INVENTOR.

EDWAgD J. HIZ/L v BY ATTORNEY PATENTEWY29|975 3.735535 SHEET 2 OF 3 FIG.3

IN VENTOR.

EDWARD J. HAZE/V ATTO'Q E Y BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to apparatus applicable to low speedflight craft for measuring airspeed magnitude and direction and, moreparticularly, to apparatus of the kind described for providing saidmeasurements directly and with increased accuracy.

2. Description of the Prior Art Low speed flight craft such as V/STOL orhelicopter craft require airspeed measuring apparatus. Prior to thepresent invention, impact pressure has been used to provide themeasurement, butthis yields indicated airspeed, and barometric pressureand ambient temperature corrections must be applied to determine trueairspeed. A prior art device of this kind is described in U. S. Pat. No.3,373,605 issued Mar. 19, 1968 to J. L. Beilman.

SUMMARY OF THE INVENTION This invention is an improvement over suchprior art devices and contemplates temperature sensors mounted toopposite ends of an arm rotating in a plane parallel to the plane offorward motion of the flight craft. As the craft moves, the sensors eachsense different instantaneous temperatures depending on the angularposition of the arm and speed of the craft. Themagnitude of thetemperature difference varies approximately sinusoidly and, if aconstant rotational speed is assumed, a direct measurement of vehicleairspeed and air direction is provided.

One object of this invention is to accurately and di rectly measure trueairspeed and air direction of a low speed flight vehicle and to avoidthe need for barometric pressure and ambient temperature corrections.

Another object of this invention is to provide said measurements byrelying on the adiabatic temperature rise due to stopping a movingstream of air.

Another object of this invention is to eliminate the need for statictemperature measurement and to utilize total temperature measurement fordetermining airspeed magnitude and direction.

Another object of this invention is to linearize the relationshipbetween airspeed and temperature measurements.

The foregoing and other objects and advantages of the invention willappear more fully hereinafter from a consideration of the detaileddescription which follows, taken together with the accompanying drawingswherein two embodiments of the invention are illustrated by way ofexample. It is to be expressly understood, however, that the drawingsare for illustration purposes only and are not to be construed asdefining the limits of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial representation of ahelicopter employing the airspeed measuring apparatus described herein.

FIG. 2 is a diagrammatic plan view showing helicopter rotor blades andtemperature sensors mounted at the ends thereof according to theinvention.

FIG. 3 is a vector diagram showing component and resultant velocities ofthe afor'enoted sensors due to the rotation of the helicopter blades andthe forward flight of the craft.

FIG. 4 is a combination block diagram-electrical schematic showing anembodiment of the invention wherein resistance thermometers are used astemperature sensors.

FIG. 5 is a block diagram of another embodiment of the invention whereinthermocouples are used as temperature sensors.

DESCRIPTION OF THE INVENTION FIG. 1 shows a low speed aircraft such as ahelicopter 2 having a body or frame 4 and a rotor 6. Rotor 6 includes apair of diametrically opposed and substantially horizontal blades 8A and8B mounted on a shaft 10 which is driven by a motor 11, such as is wellknown in the art, to rotate about axis y-y at a constant rate.Helicopter 2 is moving at a relatively low true airspeed V in a planesubstantially parallel to the plane of rotation of rotor 6. Atemperature sensor 12A is mounted at the end of blade 8A and a similartemperature sensor 12B is mounted at the end of blade 88.

As shown in FIG. 2, rotor 6 rotates at an angular velocity w and theinstantaneous angular displacement (w t) of the rotor blades from thevelocity vector of helicopter 2 is designated as a.

It is known that the adiabatic temperature rise, due to stopping amoving stream of air is a function of the velocity of that air. Therelationship may be expressed as follows:

T T (l/2 cp) V where C, is the specific heat of the air at constantpressure; T is the total temperature of the air; T, is the statictemperature and V is true airspeed.

Various means have heretofore been tried to make use of thisrelationship for measuring true airspeed but have not been successful,primarily because of the difficulty in measuring static temperature (T,The device of the invention, as will be shown, eliminates the necessityfor measuring static temperature and linearizes the relationship betweenairspeed and temperature measurements to have utility at relatively lowairspeeds.

Considering the arrangement shown in FIGS. 1 and 2, if axis y-y isstationary, then the temperatures sensed by sensors 12A and 128 will bethe same for all angular displacements a of rotor 6. However, whenhelicopter 2 and axis yy move at a velocity V, the instan'taneoustemperatures (T, and T sensed by sensors 12A and 123 respectively, willbe different depending on the angular position of rotor 6 and craftvelocity V. This is so because at one angular position of sensor 12A thecraft velocity (V) is added to the rota tionallvelocity (V w of thesensor while simultaneously the craft velocity (V) is subtracted fromthe rotation velocity (V1,, of sensor 12B.

With reference to the vector diagram of FIG. 3, wherein V is an airspeedcomponent of sensors 12A and 1213 due to the forward speed of craft 2, Vis an airspeed component due to the rotation of sensors 12A and 12B andV and V are resultant airspeeds of the respective sensors 12A and 1213,total temperature (T, in accordance with the relationship in equation(1), may be expressed as follows:

'1' T, (H2 cp)V,

'1'. T, 1/2 cp)V The difference in the total temperature A T,

measured by sensor 12A and 123, may be expressed as follows:

Substituting the values for V and V from equations (2) and (3) intoequation (1C) the following is obtained:

sinoz-l- V"')] A T=( l/2 cp) (4VV sin a) A T=(4v.

Since C, is a constant for air below a temperature of 1,000 R and sinceV.- may be held constant by design;

/2 cp) V sina AT= KV sina where K is a constant equal to (4 V /2 cp).

Thus, it is seen that true airspeed may be measured by sensing thedifference in total temperature (A T) between advancing and retreatingsensors 12A and 128, respectively, mounted to rotating blades 8A and 8B,and that the airspeed varies linearly with the temperature difference atconstant rotational velocity. Moreover, the magnitude of the temperaturedifference varies sinusoidly with angular displacement at and is at amaximum when a is 90.

With the foregoing analysis in mind, one embodiment of the invention isshown in FIG. 4, wherein temperature sensors 12A and 12B are resistancethermometers operating on the basic principle that the resistance of ametallic conductor changes with temperature. These devices generallyinclude a calibrated coil of wire,

Rosemont Engineering Co., Minneapolis, Minn., and described in theirBulletin 126027.

With reference then to FIG. 4, sensor 12A includes a resistance I iwhich, with a resistance R,,, forms two arms of a Wheatstone Bridge 20,with the remaining arms of the bridge formed by resistances R and R Ana.c. source 22 is connected at a terminal 24 intermediate resistances Rand R and is connected at a terminal 26 intermediate resistances R and RSensor 12B includes a resistance RT which, with a resistance R forms twoarms of a Wheatstone Bridge 28, with the remaining arms of the bridgeformed by resistance R and R An a.c. source 30 is connected at aterminal 32 intermediate resistances ltand and is 5 connected at aterminal 34 intermediate resistances R,

and R,.

The output of bridge 20 taken across terminals 23 and 25 intermediateresistances lt-r and R and R,, and R,,, respectively, is applied to anamplifier which provides an amplified voltage E corresponding totemperature T sensed by sensor 12A.

The output of bridge 28 taken across terminals 27 and 29 intermediateresistances R and R and lt-r and R respectively, is applied to anamplifier 42 which provides an amplified voltage E corresponding totemperature T sensed by sensor 128.

Voltages E and E: are applied to a summing amplifier 44 which sums thevoltages and provides an amplified, sinusoidal summation voltage E asfollows:

o 1 z r A"' B) i Voltage E is applied to an amplitude indicatordesignated generally by the numeral 46. Since the maximum amplitude ofvoltage E varies with temperature and hence airspeed is heretoforeshown, amplitude indicator 46 may be a conventional type voltmetercalibrated in terms of airspeed for the constant rotational speed oftenplatinum, placed in the location where the tem.

(m) of rotor 6 so that airspeed may be read directly by the pilot of thecraft.

It will now be understood that the speed of craft 2 in any directioncauses the airspeed indication from sensors 12A and 12B on rotor 6 tovary sinusoidally during each revolution of rotation. Thus, thevariation in airspeed indication during a revolution has a phaserelationship, as compared with the craft fore and aft (longitudinal)axis, to the direction of air flow. In other words, the amplitude andphase of signal E have a specific relation to the airspeed and directionof craft 2, respectively.

An indication of air direction may be obtained by comparing the phase ofsignal E with a signal having a fixed phase relationship to the craft.Accordingly, signal E is applied to a direction indicator 48 which maybe an induction type device having a pair of windings cooperating in awell known manner to position a pointer according to the phaserelationship between E and a fixed phase signal. An example of one suchdevice suitable for purposes of the present invention is i1- lustratedin U. S. Pat. No. 2,524,747 issued to W. A. Ayers, et al on Oct. 10,1950, it being understood that other devices well known in the art mayalso operate on signal E, for providing a direction indication, the samenot being the subject of this invention.

Altemately, signal E from amplifier 44 may be applied to a utilizingdevice such as a computer 49 which computes flight data therefrom foraiding the pilot in flying the craft.

In another embodiment of the invention illustrated in FIG. 5, sensors12A and 1213 may be conventional bimetallic thermocouples. Sensors ofthis type have the advantage of providing the desired information simplyand with reduced electrical circuitry.

With reference then to FIG. 5, sensor 12A includes a thermocouple 50Ahaving dissimilar metallic members 52A and 52B and sensor 12B includes athermocouple 5013 having dissimilar metallic members 54A and 548.Members 52A and 54A are connected by a conductor 56. Member 543terminates in an output conductor 58 and member 52B terminates in anoutput conductor 60, with a sinusoidal signal E corresponding toairspeed provided across conductors 58 and 60. Signal E is applied to anamplitude indicator 46A and to a direction indicator 48A for indicatingmagnitude and direction of airspeed. Alternatively, signal E may beapplied to a utilizing device such as a computer 49A.

With the above description of the invention in mind it will now be seenthat means are provided for directly measuring true airspeed,eliminating additional measurement and avoiding disadvantages of otherdevices which detract from the accuracy of the measurement. Moreover,the need for measuring static temperature is eliminated and therelationship between airspeed and temperature measurements islinearized, making the implementation simple and useful down to the verylow airspeeds. 7

Although only two embodiments of the invention have been illustrated anddescribed, various changes in the form and relative arrangements ofparts, which will now appear to those skilled in the art may be madewithout departing from the scope of the invention. Reference is,therefore, to be had to the appended claims for a definition of thelimits of the invention.

What is claimed is:

1. True fluid speed measuring apparatus comprising:

a rotatably mounted arm supported for movement in a fluid stream;

means for moving the arm in the fluid stream;

means for rotating the moving arm at a substantially constant rate;

temperature sensors mounted at opposite ends of the arm for sensinginstantaneous total temperatures in accordance with the angulardisplacement of the arm and the speed of the arm in the stream andcommensurate with the adiabatic temperature rise providing correspondingsignals; and

summing means connected to the sensors and responsive to the signalstherefrom for providing a signal corresponding to the summation of saidsignals, said summation signal being a measurement of true fluid speed.i

2. Apparatus as described by claim 1, wherein the temperature sensorsinclude:

resistance thermometers having metallic conductors of changingresistance in accordance with temperature; and

circuit means for measuring instantaneous resistance changes and forproviding the corresponding signals in accordance therewith.

3. Apparatus as described by claim 1, wherein the temperature sensorsinclude:

thermocouples having dissimilar metallic members for sensinginstantaneous temperature changes and for providing the correspondingsignals in accordance therewith.

4. Apparatus as described by claim 1, including:

indicating means connected to the summing means and responsive to thesummation signal for indicating fluid direction.

5. Apparatus as described by claim 1, including:

indicating means connected to the summing means and responsive to thesummation signal for indicating true fluid speed.

6. Apparatus as described by claim 4, wherein the summation signal is analternating signal and the indicating means includes:

means for comparing the phase of the alternating signal with a signalhaving a fixed phase relationship with the direction of movement of thearm in the fluid stream.

7. Apparatus for measuring airspeed of a helicopter type craft,comprising:

temperature sensors mounted at opposite ends of a helicopter rotor bladerotating at a substantially constant rate;

said temperature sensors being effective for sensing instantaneous totaltemperatures in accordance with the angular displacement of the rotorblade and the speed of the craft, commensurate with the adiabatictemperature rise due to movement of the blade through the air, and forproviding corresponding signals; and

summing means connected to the sensors for summing the signals therefromand for providing a summation signal, said signal being a measurement oftrue airspeed.

* I III

1. True fluid speed measuring apparatus comprising: a rotatably mountedarm supported for movement in a fluid stream; means for moving the armin the fluid stream; means for rotating the moving arm at asubstantially constant rate; temperature sensors mounted at oppositeends of the arm for sensing instantaneous total temperatures inaccordance with the angular displacement of the arm and the speed of thearm in the stream and commensurate with the adiabatic temperature risedue to movement of the arm in the stream, and for providingcorresponding signals; and summing means connected to the sensors andresponsive to the signals therefrom for providing a signal correspondingto the summation of said signals, said summation signal being ameasurement of true fluid speed.
 2. Apparatus as described by claim 1,wherein the temperature sensors include: resistance thermometers havingmetallic conductors of changing resistance in accordance withtemperature; and circuit means for measuring instantaneous resistancechanges and for providing the corresponding signals in accordancetherewith.
 3. Apparatus as described by claim 1, wherein the temperaturesensors include: thermocouples having dissimilar metallic members forsensing instantaneous temperature changes and for providing thecorresponding signals in accordance therewith.
 4. Apparatus as describedby claim 1, including: indicating means connected to the summing meansand responsive to the summation signal for indicating fluid direction.5. Apparatus as described by claim 1, including: indicating meansconnected to the summing means and responsive to the summation signalfor indicating true fluid speed.
 6. Apparatus as described by claim 4,wherein the summation signal is an alternating signal and the indicatingmeans includes: means for comparing the phase of the alternating signalwith a signal having a fixed phase relationship with the direction ofmovement of the arm in the fluid stream.
 7. Apparatus for measuringairspeed of a helicopter type craft, comprising: temperature sensorsmounted at opposite ends of a helicopter rotor blade rotating at asubstantially constant rate; said temperature sensors being effectivefor sensing instantaneous total temperatures in accordance with theangular displacement of the rotor blade and the speed of the craft,commensurate with the adiabatic temperature rise due to movement of theblade through the air, and for providing corresponding signals; andsumming means connected to the sensors for summing the signals therefromand for providing a summation signal, said signal being a measurement oftrue airspeed.